1. Field of Invention
This invention relates to air driven turbine handpieces for use in dental treatment and more particularly to air bearings for such air driven turbine handpieces.
2. Prior Art
As is well known, the cutting of teeth is one of the items of dental treatment. As shown in FIG. 1, the cutting of the teeth is carried out by a dental handpiece 1. The handpiece 1 is so constructed that a cutting tool 2 can be attached to the end of the handpiece 1 in such a manner that the tool 2 is turned at a high rate of speed to thereby cut the teeth. In order to reduce the pain for the patient and to increase durability and cutting efficiency of the cutting tool, the tool 2 must be turned at a high speed.
For safety sake, the speed of rotation of the tool 2 must be limited and therefore is limited to approximately 500,000 rpm under no load conditions. In order to meet such a demand, an air bearing journaled handpiece using an air bearing has been developed. The air bearing journaled handpiece offers the advantages that it reduces the pain to the patient and increases cutting efficiency to a greater degree than the prior art ball bearing handpieces.
A conventional prior art air bearing journaled handpiece is shown in FIG. 2. As shown in FIG. 2, the handpiece comprises a turbine rotor 3 having shaft members 32 and 32' provided on both ends of the rotor 3 and extending along the axis of rotation of the rotor 3. Blade elements 31 are integrally provided on the rotor 3 bearings 42 and 42' support the shaft member 32 and 32'. The bearings 42 and 42' are each generally fixedly supported by O-rings 51 and 52 in casing 5.
The bearing mechanism of the type described above has a disadvantage in that the bearings 42 and 42' are axially moved by the twist or dimensional error of the O-rings 51 and 52 which in turn produces non-uniformity in the pressure distribution in the gaps 34 and 34' formed between the shaft member 32 and 32' of the turbine rotor 3 and the bearings 42 and 42' and affect a reduction in the load carrying capacity of the bearings 42 and 42'. Since the axial length of the gaps 34 and 34' is relatively short, the pressure distribution in each of the gaps 34 and 34' becomes parabolic in the axial direction of the gaps 34 and 34'. As a result, the load carrying capacity of the bearing is further reduced and therefore even the smallest load torque reduces the number of rotations of the tool 2.
To overcome these disadvantages, the present applicant filed an application in the Japanese Patent Office on Dec. 28, 1972 which disclosed a turbine engine driven air bearing handpiece for dental treatment designed to overcome these disadvantages. Such Japanese patent was registered on Apr. 28, 1977 and has a Japanese Pat. No. 856133. This handpiece is shown in FIG. 3.
As shown in FIG. 3, the shaft member 32 is in connected to only one side of the blade element 31 and no shaft member is provided on the other side of the blade element 31. Namely, the shaft member 32 is journaled by air bearings in the direction of thrust between a radial gap Rg and thrust gaps Tg and Tg' formed in pairs forwardly and rearwardly of the blade element 31. The radial gap Rg is formed between the bearings 42 provided by O-rings 51 inside the casing that correspond with the shaft member 32. According to the construction described, an ideal number of rotations (more than 400,000 rpm) for tooth cutting can be obtained and furthermore even if there is twist or a dimensional error in the O-rings 51, such twist or error ends in only one side and there is no possibility of the axis of rotation of the shaft member 32 moving during rotation of the shaft member 32. As a result, the bearing mechanism for a dental handpiece capable of reducing the disagreeable feeling and pain for the patient and having excellent rotational characteristics is achieved.
However, as a result of this construction, the air bearing mechanism makes it impossible to enlarge the thrust gaps Tg and Tg' as an area of bearing with respect to the direction of thrust. Accordingly a new problem has been created. In operation when the operator depresses the cutting tool strongly to drill a tooth, the area above the bearings cannot sufficiently resist the axial load in the direction of thrust. Accordingly, the thrust bearing capacity based exclusively on the air pressure inside the gaps Tg and Tg' cannot resist the load and consequently the rear end surface 312 of the blade element 31 is brought by rotation into contact with the inside wall surface of an end cover 53. As a result friction is created and the life of the handpiece 1 and the number of rotations of the turbine rotor 3 is decreased when the tool 2 is put under load. Therefore the cutting tool 2 which is coupled to the turbine rotor 3 and rotates in synchronism thereto receives these adverse effects and turns at a lower rpm.